PortableRid is wrong when not supplied as an arg

https://github.com/dotnet/scenario-tests/pull/189 adds the `--portable-rid` CLI option which will be supplied when running the tests inside the VMR. When running the tests outside of the VMR, i.e. in scenario-tests CI, that option isn't provided and a default value of `linux-` will be used. That default already existed before so that change doesn't make this worse.

We should remove this default eventually by calculating the portable RID when unset either in the Arcade SDK (preferred so that it's in a central place) on inside this repository.